1. Field of the Invention
The present invention relates to a magnetic detection apparatus which detects a magnetic field intensity based on a magnetic moving object, whereby the rotation number or rotation angle of a rotating object, for example, can be detected. More particularly, it relates to a magnetic detection apparatus which can detect the moving direction of a magnetic moving object.
2. Description of the Related Art
In a magnetic detection apparatus which detects a magnetic field intensity based on a magnetic moving object, a scheme for detecting the moving direction of the magnetic moving object embraces several related-art techniques, depending upon a signal which a magnetic detection sensor outputs, and a method by which a computer unit processes the signal. In a case where the sensor output signal is a digital signal (rectangular wave), the detection of the moving direction of the magnetic moving object can be broadly classified into a scheme which time-modulates the sensor output signal, and a scheme which potential-modulates the sensor output signal. In the scheme which time-modulates the sensor output signal, the output timing (time) of the magnetic detection sensor differs in accordance with the moving direction of the magnetic moving object. By way of example, the time interval of the signal is denoted by a when the moving direction is a forward direction, and the time interval of the signal is denoted by b (≠a) when the moving direction is a reverse direction. Then, the moving direction can be detected in such a way that the computer unit measures the time interval of the sensor output signal.
With this scheme, in a case where the moving direction has changed during the time intervals a or b, the detection of the moving direction delays in principle. On the other hand, in the scheme which potential-modulates the sensor output signal, the signal potential (high level or low level) of the magnetic detection sensor differs in accordance with the moving direction of the magnetic moving object. By way of example, the low level of the signal at the time when the moving direction is a forward direction is denoted by S[low-1], and the low level of the signal at the time when the moving direction is a reverse direction is denoted by S[low-2] (≠S[low-1]). Then, the moving direction can be detected in such a way that the computer unit measures the potential of the sensor output signal. (Refer to, for example, Patent Document 1 being JP-A-2002-90181.)
The present invention belongs to the scheme which potential-modulates the sensor output signal. More specifically, as disclosed in Patent Document 1, the scheme includes a magnet which generates a bias magnetic field, and a magnetic detection sensor in which first and second magnetoresistive elements opposed to a magnetic moving object and juxtaposed in the moving direction thereof, and the output circuits of the respective elements are built, wherein a magnetic field change corresponding to the movement of the magnetic moving object is detected by then, and the moving direction is also detected by setting at least either of the high level and low level of an output signal at different potentials in accordance with the moving direction.
In the related-art technique stated in Patent Document 1, however, a method for processing the sensor signal is not optimal in the detection of the magnetic moving object, and delays occur in some cases. The detection delays are apprehended to cumulate in the repeated operations of a forward direction and a reverse direction in the moving direction, and the related-art technique is not suited to a use in which a movement position needs to be accurately detected by the detection of the moving direction.
As the application field of the magnetic detection apparatus, there is, for example, an on-vehicle rotation detection apparatus which detects a rotation number or a rotation angle in a vehicle so as to control an engine or a transmission. In general, the on-vehicle rotation detection apparatus is subject to a severe mounting environment, and a rotating direction is a forward rotation in most cases. Therefore, a position signal especially during the forward rotation needs to have not only a controllability, but also a noise immunity and robustnesses for a temperature change as correspond to a wide range of detection conditions (the rotation number, etc.).
In actuality, filters are often incorporated into, for example, the output of the magnetic detection sensor and the input of the computer unit as noise countermeasures. Therefore, a signal waveform becomes dull in the rise and fall of the rectangular wave, to pose the problem that the rectangular wave cannot sufficiently rise or sufficiently fall in a high rotation mode. In this case, the amplitude of the sensor output signal in the forward rotation mode should desirably be enlarged beforehand. Also from the viewpoint of the noise immunity, it is important that the amplitude of the sensor output signal in the forward rotation mode is made sufficiently large, thereby to ensure an allowance degree at which the hysteresis of a comparison threshold value signal in the comparator circuit of the computer unit can be set large. That is, in the use of the on-vehicle rotation detection apparatus or the like, the condition for avoiding the risk in practical use exists for the signal in the forward rotation mode.